This invention relates to multi-junction photovoltaic cells that are particularly useful in non-terrestrial applications.
Photovoltaic cells, which are devices that convert light energy into electrical energy, are known. Where the cells are employed in non-terrestrial applications, e.g., satellites, the efficiency of the cell is of paramount importance because the cost of delivering objects into orbit is directly related to the payload weight. In other words, the number of cells required to power any given satellite can be decreased if the efficiency of the photovoltaic cell used to power the satellite is increased. This reduction in the number of cells employed directly translates into a decreased payload weight, which makes the deployment of the satellite or other non-terrestrial device more efficient.
A known technique for increasing cell efficiency includes constructing a multi-junction cell. Each cell within a multi-junction cell can convert light energy of a different wavelength and thereby attempt to maximize the available solar spectrum. The construction of multi-junctions cells, however, offers many challenges, especially where the cell is constructed by using epitaxial methods. To begin with, efficiency of a multi-junction cell depends on the ability to provide a single crystal lattice that is substantially free of defects; i.e., the cell should be monolithic. Defects in the lattice structure of the cell cause loss of useful minority carriers through recombination and absorption. Also, the selection of an appropriate lattice constant and band gap of the epitaxial layers constituting the cell directly impacts cell efficiency. Further, the cell should be constructed to maximize current flow through each subcell. And, the individual subcells must not only be lattice matched with each other, but they should be individually engineered at an appropriate band gap to maximize absorption.
While various cell structures have been made, a triple-junction cell having a theoretical efficiency of about 31% at airmass zero one-sun condition (AM0 1-sun) and a practical efficiency of about 27% (AM0 1-sun) is believed to be the most efficient cell that has been constructed in the prior art. This cell includes a 1.85 eV InGaP top cell, a 1.43 eV GaAs middle cell, and a 0.67 eV Ge bottom cell, where each cell is constructed at lattice constant of 5.65 xc3x85.
Because efficient deployment of non-terrestrial devices, such as satellites, is becoming increasingly desirous, there is a need to provide a photovoltaic cell for use in non-terrestrial devices that has even greater efficiency than those proposed. Even the smallest incremental increase in efficiency can translate into tremendous efficiency in the overall deployment of the non-terrestrial device.
In general the present invention provides a triple-junction photovoltaic device comprising a p-doped substrate comprising GaAs, a p-doped buffer layer deposited on said substrate, where said buffer layer comprises In0.13Ga0.87As, a bottom subcell deposited on said buffer layer, where said bottom subcell includes a p-doped back window layer comprising In0.62Ga0.38P, a p-doped base layer deposited on said back window layer and comprising In0.13Ga0.87As, an n-doped emitter layer deposited on said base layer and comprising In0.13Ga0.87As, and a n-doped front window layer comprising In0.62Ga0.38P, a lower tunnel junction deposited on said bottom subcell, where said lower tunnel junction includes an n-doped lower layer comprising In0.62Ga0.38P, and a p-doped upper layer comprising Al0.09Ga0.91As, a middle subcell deposited on the upper surface of said lower tunnel junction, where said middle subcell includes a p-doped back window layer comprising In0.62Ga0.38P, a p-doped base layer deposited on said back window layer and comprising In0.49Ga0.51As0.23P0.77, an n-doped emitter layer deposited on said base layer and comprising In0.49Ga0.51As0.23P0.77, and a n-doped front window layer comprising Al0.42In0.58P, a upper tunnel junction deposited on said middle subcell, where said upper tunnel junction includes an n-doped lower layer comprising In0.62Ga0.38P, and a p-doped upper layer comprising Al0.09Ga0.91As, a top subcell deposited on the upper surface of said upper tunnel junction, where said top subcell includes a p-doped back window layer comprising Al0.42In0.58P, a p-doped base layer deposited on said back window layer and comprising (In)0.6(Ga0.33Al0.67)0.4P, an n-doped emitter layer deposited on said base layer and comprising (In)0.6(Ga0.33Al0.67)0.4P, and a n-doped front window layer comprising Al0.42In0.58P, and an n-doped cap layer deposited on said top subcell and comprising In0.13Ga0.87As.
The present invention also includes a solar cell comprising a first subcell, where the first subcell includes a doped base layer and an emitter layer that is oppositely doped from the first base layer, and where the subcell comprises InGaAs, a second subcell, where the second subcell includes a doped base layer and an emitter layer that is oppositely doped from the base layer, and where the second subcell comprises InGaAsP, a third subcell, where the third subcell includes a doped third base layer and an emitter layer that is oppositely doped from the third base layer, and where the third subcell comprises AlGaInP, where said first subcell, said second subcell, and said third subcell are lattice matched.
The present invention further includes a multi-junction photovoltaic device comprising a bottom subcell, where said bottom subcell includes a p-doped base layer comprising germanium, and an n-doped emitter layer comprising germanium, a bottom window layer deposited on said bottom subcell, where said bottom window layer comprises GaAs, a p-doped buffer layer deposited on said bottom window layer, where said buffer layer comprises In0.13Ga0.87As, a lower tunnel junction deposited on said buffer layer, where said lower tunnel junction includes a degenerately n-doped lower layer comprising In0.13Ga0.87As, and a degenerately p-doped upper layer comprising In0.13Ga0.87As, a lower-middle subcell deposited on the upper surface of said lower tunnel junction, where said lower middle subcell includes a p-doped back window layer comprising In0.62Ga0.38P, a p-doped base layer deposited on said back window layer and comprising In0.13Ga0.87As, an n-doped emitter layer deposited on said base layer and comprising In0.13Ga0.87As, and a n-doped front window layer comprising In0.62Ga0.38P, a middle tunnel junction deposited on said lower middle subcell, where said middle tunnel junction includes an n-doped lower layer comprising In0.62Ga0.38P, and a p-doped upper layer comprising Al0.09Ga0.91As, an upper-middle subcell deposited on the middle tunnel junction, where said upper-middle subcell includes a p-doped back window layer comprising In0.62Ga0.38P, a p-doped base layer deposited on said back window layer and comprising In0.49Ga0.51As0.23P0.77, an n-doped emitter layer deposited on said base layer and comprising In0.49Ga0.51As0.23P0.77, and a n-doped front window layer comprising Al0.42In0.58P, an upper tunnel junction deposited on said upper-middle subcell, where said upper tunnel junction includes an n-doped lower layer comprising In0.62Ga0.38P, and a p-doped upper layer comprising Al0.09Ga0.91As, a top subcell deposited on the upper tunnel junction, where said top subcell includes a p-doped back window layer comprising Al0.42In0.58P, a p-doped base layer deposited on said back window layer and comprising (In)0.6(Ga0.33Al0.67)0.4P, an n-doped emitter layer deposited on said base layer and comprising (In)0.6(Ga0.33Al0.67)0.4P, and a n-doped front window layer comprising Al0.42In0.58P, and an n-doped cap layer deposited on the top subcell and comprising In0.13Ga0.87As.
The photovoltaic cells of the present invention advantageously provide improvements over the prior art photovoltaic cells in overall cell efficiency and power density. Also, the cells of this invention exhibit useful radiation hardness. As a result, the cells of this invention are particularly useful in non-terrestrial applications because they can provide overall efficiency in the deployment of non-terrestrial devices that employ the cells as an energy source. In one embodiment of this invention, a triple-junction photovoltaic cell provides a theoretical efficiency of about 36% (AM0 1-sun) and a practical efficiency of about 31% (AM0 1-sun). In another embodiment, a quad-junction photovoltaic cell provides a theoretical efficiency of about 40% (AM0 1-sun) and a practical efficiency of about 34% (AM0 1-sun). Also, the cells of this invention can advantageously be current matched by selecting an appropriate thickness for each of the various subcells.